1. Field of the Invention
The present invention relates to heteroaromatic compounds with nonlinear optical properties. In particular, the present invention relates to nonlinear optical materials having highly conjugated structures with two or more aromatic rings, at least one of which is a five-membered heteroaromatic ring. The compounds of the present invention, once suitably oriented, are capable of highly efficient second harmonic generation (SHG) and electro-optic modulation of an electromagnetic wave whose wavelength may be between 300 nm and 2000 nm. The present invention further relates to the incorporation of the compounds of the present invention into polymeric matrices, including polymers having side chains of the disclosed compounds.
2.Description of the Prior Art
High efficiency nonlinear optical (NLO) materials capable of doubling or tripling the frequency of incident light are currently of great scientific and technological interest for use in optical telecommunications, signal processing and the construction of optical computers. Nonlinear optics is concerned with the interaction of electromagnetic fields in various media to produce new fields which may be altered in phase, frequency or amplitude. The NLO effect of a material upon an electromagnetic field is a function of the second and higher order terms of the following equation: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 +
P is the polarization of the material, E is the intensity of the electric field, and the coefficients alpha, beta, gamma, etc. are indicative of the NLO susceptibility of the material. Such coefficients are constant for a given material, but vary from material to material. The second order coefficient, beta, for a given material, is indicative of the SHG properties of the material, with SHG efficiencies increasing as the value of beta increases.
Candidate NLO materials should possess good physical properties, such as high optical transparency, low dielectric constant, high laser damage threshold, good solubility in the solvents used for spin casting of optical materials, and the like. The materials should also possess the mechanical and thermal properties required of optical materials, in particular, high beta values, fast response times and nonlinear susceptibility over a broad range of wavelengths, particularly at wavelengths between about 300 nm and about 2000 nm.
The first NLO materials were monocrystal minerals such as KH.sub.2 PO.sub.4, LiNbO.sub.3, InSb and NH.sub.4 H.sub.2 PO.sub.4. However, these materials are costly to grow in high optical quality and show relatively low SHG properties. These materials are expected to be replaced by organic and polymeric materials with large delocalized pi-electron systems, which not only exhibit greater nonlinear susceptibilities, but also can be varied to optimize the desired physical and mechanical properties.
Early organic NLO materials were based upon conjugated pi-electron chromophores with charge asymmetry, such as 4-dimethylamino-4-nitrostilbene (DANS),dissolved in a suitable polymer matrix, which are disclosed by Williams, Angew. Chem. Int. Ed. Engl., 23, 690-703 (1984). However, such combinations were of limited solubility, resulting in crystallization of the guest chromophore molecule out of the host matrix, or mobility of the guest molecules in the matrix, resulting in a loss of SHG performance. Such materials are also exemplified by U.S. Pat. No. 4,892,681 to Miyata and U.S. Pat. No. 4,894,186 to Gordon.
The insolubility of these materials in a polymer matrix was overcome by the covalent linking of the NLO chromophores to the polymer backbones. This is also disclosed by Williams, as well as U.S. Pat. Nos. 4,894,263 to Dubois, 4,933,112 to DeMartino and 4,935,292 to Marks. These references disclose polymers having NLO chromophore side chains of a series of aromatic rings separated by pi-electron conjugated carbon-carbon, carbon-nitrogen and nitrogen-nitrogen bridges. The side chains utilize from two to four or greater aromatic ring/conjugated bridge combinations. The aromatic rings disclosed are based on six-membered rings such as benzene and pyridine. The chromophore side chain is covalently attached to the polymer by a reactive binding, typically a long chain alkyl group linking one of the aromatic rings of the chromophore to a reactive functional side chain of the polymer. To induce charge asymmetry, and consequently second order nonlinear polarizability, the aromatic ring attached to the reactive binding is ring-substituted with an electron donating group while the aromatic ring at the end of the chromophore is ring-substituted with an electron accepting group, and the dipoles of the chromophores are aligned, in accordance with the method described by Williams and by U.S. Pat. No. 4,935,292.
Some azomethine derived chromophores, which contain five-membered heteroaromatic rings, are disclosed as having third order NLO susceptibilities by Dirk, Proc. SPIE-Int. Soc. Opt. Eng., 1147, 18-25 (1989). However, the reported third order susceptibility of these materials is low, and second order properties cannot be reasonably predicted from third order susceptibilities, let alone from low-value third order susceptibility.
Methods by which polymers having chromophore side chains may be prepared vary. U.S. Pat. No. 4,933,112 discloses the attachment of the chromophore to a monomer that is then polymerized. U.S. Pat. No. 4,935,292 discloses the attachment of the chromophore to a functionalized polymer.
U.S. Pat. No. 4,894,263 discloses that either method of attachment may be used depending on the constituents of the material. It is further disclosed that the constituents of the chromophore may be assembled in one or more steps.
As noted above, the chromophores are covalently linked to polymer backbones because of their limited solubility in polymer matrices. However, the synthesis of polymers having polar side groups such as chromophores possessing a high degree of charge asymmetry can be problematic. The limited solubility is also exhibited by the chromophores and monomer-linked chromophores in polymerization solvents, and by the chromophores in reaction solvents for linking monomers and chromophores. The electron accepting groups of chromophores possess reactivity toward radicals that, together with the low monomer solubility, results in low yields of polymers having low broad molecular weights (typically less than 15,000 daltons), low T.sub.g 's and low chromophore incorporation. Among the number of electron-acceptor groups employed with NLO-chromophores is the tricyanovinyl functional group, which is preferred because it induces a large degree on nonlinearity. However, it is highly susceptible to polymerization conditions. This limits the incorporation of this functional group into polymers containing NLO-chromophores.
The tricyanovinylation of poly (N-vinylindole) and poly (3-vinylcarbazole) at attachment yields of 35% to 50% have been reported. Oshiro et als., Polym. J., 6(5), 364-9 (1974) disclose the tricyanovinylation of poly (N-vinylindole) and U.S. Pat. No. 3,978,029 discloses the tricyanovinylation of poly (3-vinylcarbazole). In both publications the base polymer is reacted with tetracyanoethylene in N,N-dimethylformamide (DMF) at 50.degree.-140.degree. C. Yields of 35% to 50% have not been achieved to date with typical NLO chromophores.
Once polymerized, the polymer is spin-cast to form a film, which is then heated to near its glass-rubber transition temperature (T.sub.g) to enhance molecular motion, including rotation of the chromophore side chains. An intense electric field is then applied to the heated film for a given length of time and the film is then cooled to well below the Tg in the presence of the electric field. This results in alignment of the dipoles of the side chains, providing a system in which the NLO components are locked in a preferred orientation while at the same time covalently linked within a polymer matrix. According to U.S. Pat. No. 4,935,292, NLO efficiency can be increased by repeatedly heating the material above and then cooling it below the Tg several times prior to applying the electric field. It is disclosed that this reduces the number of pinholes, voids, free volume and other anomalies that can cause short circuits during poling, and also removes residual stress from the film.
Notwithstanding these advances, there remains a need for more efficient second-order nonlinear optically active materials and methods for preparing same.